Re: [ATM] The 15.5 Ultrathin Project, Astigmatism discussion

["Peter" <peter@kitgear.com>]

David,

Please describe the mirror support used for the fine grinding and polishing
stages. Photos would be good. For example how do you know you have good
contact when applying polishing or fine grinding pressure? My one ultra-thin
mirror looked very much like your photos and I had to regrind the back flat
and regrind the front then polish to make a good mirror.

Also please describe your method so we can better understand. How often did
you rotate the mirror and tool? Also how heavy is your tool relative to the
mirror?

Thanks,

Peter



-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net]On Behalf
Of vorblesnak@peak.org
Sent: January 19, 2006 1:55 PM
To: atm@atmlist.net
Subject: [ATM] The 15.5 Ultrathin Project, Astigmatism discussion


I have posted an analysis of what I think is going on here ..

http://www.atmlist.net/contrib/vorblesnak-at-peak-dot-org/Ronchi%20Pictures/
Astigmatism%20Discussion.jpg

I welcome any input on that analysis that anyone wants to add.

I mounted the glass on the wooden base with double stick tape at the
support points suggested by plop.  Using an 8 inch tool I polished for
about a half an hour.  Basic W stroke, tool on top.  I begin to see
improvement and I have hopes for the figure.  I suspect the glass is so
thin that any heat will distort it and mess up the figure.

Two possible directions of work come to mind.  One is to continue with the
sub diameter tool, tool on top.  The other would be to use the full
diameter tool and put the mirror on the top.  OR I could blend the two
approaches.

The sub diameter tool is easier to use and control.  The full diameter
tool, mirror on top, would, by virtue of the stroke etal, return the
surface to a sphere almost guaranteed.

Discussion??

Specs for those new to the topic ..
15.5 inch diameter
3/16ths plate glass, slumped to an F6 curve
true focal length, 73.5 inches

David Davis
Toledo, OR 97391

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